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Glossary of Geological Terminology from Quartz 'N' All Category: Earth and Environment > Geology Date & country: 15/11/2007, UK Words: 308

Bakhchisaraian
A palaeocene / eocine succession in the former USSR covering parts of the thanetian and Ypresian

Bala
The youngest sub-period of the ordovician 463.9 - 439.0 Ma

Balas ruby
A red gem variety of spinel.

Ball and pillow structure
A structure caused by the wet sediment deformation of interbedded sand and mud, characterized by globular protrusions and isolated pillows of sandstone which form by differential settling of unconsolidated sand into mud.

Ball clay ( pipe clay)
A fine-grained clay comprising of up to 70% kaolinite plus illite, quartz, montmorillonite, chlorite and 2 - 3% carbonaceaous material, important to the ceramics industry.

Ballas
A variety of industrial diamond in the form of very hard, dense, globular aggregates of minute concentric diamond crystals.

Ballstone
A sedimentary rock comprising subspherical nodular masses, usually calcareous in an argillaceous matrix

Banakite
A potassic volcanic rock similar to shoshonite but with more silica

Bar
1. The non-si unit of pressure, equivalent to 10 Pascals. 2. A linear deposit of sand/gravel generally parallel to subparallel to a coastline or river channel.

Barker Index of Crystals
A once popular method of identifying crystals by measurement of interfacial angels.

Barranca
A steep-sided gully formed by erosion, similar to a donga.

Barremian
A stage of the cretaceous 131.8 - 124.5 Ma

Barrovain metamorphism
Regional metamorphism first recognised in Scotland in which zones of increasing metamorphic grade are characterised by the appearance of a suite of index minerals; chlorite, biotite, garnet, staurolite, kyanite, sillimanite.

Bartonian
A stage of the Eocene 42.1 - 38.6 Ma

Bashkirian
An epoch of the carboniferous 322.8 - 311.3 Ma

Basic
(of igneous rocks) containing 45-55% silica

Basin Groups 1 - 9
An era of precambrian 4150 - 3950 Ma

Batholith
A large composite intrusion with a surface area >100 km' made up of multiple plutons of gabbroic to granitic composition.

Batholith
Large intrusion, often with no visible floor, up to 1000kmSquared (386sq mi) in area

Bathonian
A stage of the Jurassic 166.1 - 161.3 Ma

Becke Line
The bright line at the margin of a grain visible when viewed in thin section which can be used to determine the refractive index relative to another grain or the mounting medium.

Bedding
Layering in the rock that can be picked out by varying textures, colours and mineralogy's. It reflects episodes of rapid deposition followed by relatively little. Beds vary in thickness, those ranging in thickness from several metres down to centimetre size are called strata. Those less than a centimetre thick are called laminations. The surface between the layers is called the bedding plane.

Bedrock
Unweathered rock beneath unconsolidated material

Berruasian
The lowest stage of the cretaceous 145.6 - 140.7 Ma

Bioclast
A grain in a limestone consisting of the skeletal material of orgainsims.

Biogenic
Description of material produced by organisms or their activities.

Bioherm
Mound of organic remains in a reef, also called areef knoll

Biostratigraphy
The subdivision and correlation of sequences of strata using fossils.

Biostrome
Laterally extensive reefs made up of shells

Bladed
(see habit) Flattened like a knife blade, as in Kyanite.

Botryoidal
(see habit) like a bunch of grapes, as in chalcedony.

Brittle
Relates to tenacity. Most minerals are brittle, and will crumble when hit, as in iron pyrites.

Cementation
Deposition of cement (usually silica, calcite or iron hydrates) between the particles in a loose sediment, thus converting it to hard rock.

Chatoyancy
Light effect which resembles the slit-eye of a cat. Arises through reflection of the light from parallel fibres, needles and cavities.

Clay Minerals
Fine-grained, hydrous silicates formed by extensive weathering of other silicate minerals.

Cleavage
Due to the internal arrangement of atoms, many minerals tend to break along a well defined set of lines. For further details, see the Fact File page.

Colour
Can be used as a guide in the field but not consistent as many minerals will show a variety of colours depending upon the impurities contained.

Columnar Joints
Vertical joints common in basalt producing regular, hexagonal pillars of rock.

Conchoidal
A distinctive fracture pattern and one displayed by broken glass. The mineral breaks with a concave or convex surface often showing concentric ridges looking like growth lines on a shell.

Concretion
Irregular or rounded mass formed by the precipitation of a new mineral within a sedimentary rock. Rounded forms are called nodules.

Contact - thermal Metamorphism
This process occurs around igneous intrusions where the heat of the intrusion 'bakes' the surrounding country rock to form a contact metamorphic aureole.

Contact twins
Formed by contact across a flat surface

Craton
Area of the continental crust that has not been significantly deformed typically since Cambrian times. Cratons contain extensive crystalline basement rocks called shields.

Cross-stratification
Phenomenon occurring in ripples, sand waves and dunes where individual strata are laid down at an angle to the main bedding planes. Also known as cross, current or false bedding.

Crystal Systems
The symmetry of crystals fall into seven distinct groups. Crystals can show one or more planes and/or axes of symmetry. The seven groups are Cubic; Tetragonal; Orthorhombic; Monoclinic; Triclinic; Hexagonal & Trigonal. For further details see the Fact File page.

Crystals
Most minerals that grow without interruption will form crystals. Crystals are mainly symmetrical solids bound by a flat (or planar) face. The crystal shape is formed according to the internal arrangement of the atoms.

Cubic Crystal System
( see above) Crystals that form the cubic system all have 4 three fold axes, 3 reference axes all at right angles to each other and of equal length.

Dendritic
(See habit) Branching treelike or moss like, as in native copper.

Detrital Deposits
Sediments (of sedimentary rocks) formed by the breakdown of existing rocks.

Diatom
Single-celled plant found in oceans and lakes with a skeleton composed of silica.

Druse
Irregularly shaped cavity containing minerals that project inwards from the cavity walls.

Dune Bedding
Large-scale cross stratification formed by the movement of sand dunes, usual several metres high.

Dyke
Vertical or near-vertical igneous intrusion that cuts across existing bedding or foliation in the host rock.

Dynamic Metamorphism
The alteration of rocks under high pressures but lower temperatures.

Earthy
Minerals that do not show a lustre are said to be earthy, or dull.

Elastic
Related to tenacity. A bent portion will spring back when pressure is released. Mica flakes are flexible and very elastic whereas talc is is flexible but inelastic.

Essential Minerals
Minerals which define a rock and must be present in order to give it a particular name.

Evaporites
Sedimentary rocks formed by the precipitation of salts in hot dry regions from shallow seas or lakes.

Even
Refers to the type of fracture of a mineral. Means that the fracture surface will be flattish, as in garnet.

Extrusive Igneous Rocks
Formed from magma that is ejected from the crust, i.e. from volcanoes, cooling on the surface.

Facies
Group of metamorphic rocks formed under a particular set of physical conditions from any rock type. Rocks of the greenschist, amphibolite and granulite facies are all formed at moderate pressures (depths of 10 - 30km), but at low, medium and high temperatures respectively.

Fault
Break in the rocks resulting in displacement either side.

Feldsparthoids
Aluminosilicate minerals, similar to feldspars but containing less silica.

Felsic
(Of rocks) essentially composed of the light coloured minerals feldspar and either feldspathoid or silica.

Ferromagnesian
Magnesium and iron bearing materials

Fibrous
( See habit ) Fine, thread like strands, as in asbestos.

Flexible
Relates to tenacity. Flexible refers to thin sheets of a mineral that will bend, as in talc.

Flow Texture
Alignment of needle-like or tabular crystals, formed when a crystallisation occurs when the molten rock is still flowing.

Fluorescence
Some minerals will glow in the dark when exposed to ultraviolet light or X-rays. This effect derives its name from the mineral fluorite, which is a good demonstrating mineral for this effect.

Foliation
The re-crystallisation of minerals under pressure causes the grains in the rock to be arranged in layers.

Form
(See also habit) Form is the term for the set of faces produced by the symmetry of a crystal. Crystals of the same mineral can take different forms. For example, pyrite can produce both cubic and octahedral crystals. If the form totally encloses a space, for example, a cube, it is said to be closed. If not, for example a prism, it is said to be open. Open forms must always be combined with other forms.

Fossil fuels
Elements like coal, oil and natural gas. They are known as fossil fuels as they are formed from organic chemicals, those that were once living matter.

Fracture
The fracture is seen when a mineral will not split cleanly on any cleavage level. Irregular breakage results where this occurs. For example, a pane of glass or quartz will form a conchoidal fracture. With the window pane you would see signs of shell like rounded fracture patterns. For more in depth details, see the Fact File page.

Fracture
Refers to the shape of the broken surface of the mineral. Can occasionally be used in identification. Can be Conchoidal, Even, Uneven & Hackly

Fumarole
Small volcanic vent issuing gasses but no lava.

Geode
A former cavity in the rock ( up to approx 1 metre in diameter) with crystal accumulations or growths on the walls.

Gneisses
Coarse grained metamorphic rocks

Graded Bedding
With coarse grains at the bottom of the bed, grading to finer particles at the top.

Granitoid
Group of plutonic igneous rocks containing quartz. Includes granodiorite and tonalite.

Granular Texture
Comprising minerals of roughly equal size.

Graphic Texture
Intergrowth of minerals, especially quartz and feldspar, resembling runic inscriptions.

Habit
Refers to the appearance of single crystals or crystalline aggregates that do not show perfect crystal shapes. There are different types of habit coming under the headings of prismatic, Acicular, Bladed, Botryoidal, Dendritic, Fibrous, Mammilated, Massive, Radiating, Reniform and Tabular.

Hackly
A term relating to the fracture pattern of a mineral. It indicates that the minerals surface is covered with small, jagged points, as in native copper.

Hardness
A measure of a crystal or mineral to scratching or abrasion. (See Moh's Scale)

Hexagonal
( see crystal systems) Crystals forming under this system have 1 vertical 6 fold axis of symmetry 3 horizontal reference axis, equal in length and at 120 degrees to each other.

Hydrothermal Deposit
Minerals which crystallise from hot watery fluids in the late stages of formation of igneous rocks.

Hydrothermal solutions
Hot fluids that result from cooling magma that react with host rocks to form replacement deposits or precipitate into cracks an fissures to form veins

Idiochromatic
Refers to minerals that are always the same colour. i.e. Malachite is always green, Azurite is always blue and Sulphur is always yellow.

Igneous Rocks
e.g. basalts and granites. Produced from hot magma rising up from the lower crust or upper mantle, which then cools and solidifies.

Intermediate
(Of Igneous rocks) With a silica content of 52 - 65% including syenite, trachyte, diorite and andesite.

Interpenetrant twins
Twins where the halves appear to have grown within each other and therefore cannot be separated.

Intrusive Igneous Rocks
Form from magna solidifying in the ground

Joints
Cracks and fissures in rocks with no displacement either side.

Labradorescence
Play of colours in metallic hues, especially in labradorite.

Laminations
Fine layering in sediments, only millimetres thick. Often forms the internal structure in beds. See bedding

Leucocratic
Rocks containing mainly light coloured or felsic minerals.

Lithic
Pertaining to rocks.

Lithification
A process whereby sediments are buried, compacted and converted into rocks.

Lustre
The amount and quality of reflected light coming from the surface of a mineral. It can be metallic, non-metallic,sub-metallic, adamantine, vitreous, subvitreous, resinous, pearly or silky

Mafic
(Of rocks) Essentially composed of dark coloured minerals.